How to fix a split Fusion Drive

If your Fusion Drive appears as two drives instead of one in the Finder, it's no longer working as a Fusion Drive. Here's how to fix it.

Fusion Drive, a storage option on some iMac and Mac mini computers, combines a hard drive and flash storage in a single volume for improved performance and storage capacity. If your Fusion Drive appears as two drives instead of one in the Finder, it's no longer working as a Fusion Drive. This can happen after replacing either drive of your Fusion Drive, or using software to intentionally split them into separate volumes.

You can continue using the two drives independently, or follow these steps to regain the benefits of having the single logical volume of a Fusion Drive.

Before you begin

If you're not sure that your Mac was configured with a Fusion Drive, or that the drive has been split:

Disconnect any external storage devices from your Mac.

Choose Apple () menu > About This Mac, then click Storage.

If you see a drive labeled Fusion Drive, your Fusion Drive is working and this article doesn't apply to you.

If you have a Fusion Drive that has been split, you should see two drives. One of them should be labeled Flash Storage, with a capacity of 24GB, 32GB, or 128GB. The other should be at least 1TB.

Use Terminal to create a Fusion Drive again

These steps permanently delete all data stored on the drives that make up your Fusion Drive. Make sure that you have a backup before continuing.

If you're using macOS Mojave

Turn on your Mac, then immediately press and hold Command-R to start up from macOS Recovery. Release the keys when you see the Apple logo or spinning globe.

When you see the macOS Utilities window, choose Utilities > Terminal from the menu bar.

Type diskutil resetFusion in the Terminal window, then press Return.

Type Yes (with a capital Y) when prompted, then press Return.

When Terminal indicates that the operation was successful, quit Terminal to return to the macOS Utilities window.

If you're using macOS High Sierra or earlier

Turn on your Mac, then immediately press and hold Command-R to start up from macOS Recovery. Release the keys you see the Apple logo or spinning globe.

When you see the macOS Utilities window, choose Utilities > Terminal from the menu bar.

Type diskutil list in the Terminal window, then press Return.

Terminal displays a table of data about your drives. In the IDENTIFIER column, find the identifier for each of the two internal, physical drives that make up your Fusion Drive. Usually the identifiers are disk0 and disk1. One of them should be 128GB or less in size. The other at least 1TB in size.

Type the following command, replacing identifier1 and identifier2 with the identifiers you found in the previous step. Then press Return.

diskutil cs create Macintosh\ HD identifier1identifier2

Example: diskutil cs create Macintosh\ HD disk0 disk1
If you get a disk unmounting error, enter diskutil unmountDisk identifier, using the first identifier you gathered previously. Then enter same command again using the second identifer.